The conventional process for tempering glass sheets involves heating a glass sheet within a furnace to a temperature range of about 1100 to 1250 degrees Fahrenheit and then transferring the heated glass sheet to a quench unit where jets of pressurized quenching air, or an equivalent gas, are impinged against its opposite surfaces. Such quenching rapidly cools the outer surfaces of the glass sheet faster than its center so that upon complete cooling the surfaces are subjected to compressive stresses while the center is tensioned. Due to the compressive stresses at the surfaces, tempered glass sheets are much more resistant to breakage than annealed glass. Also, upon breakage, tempered glass sheets shatter into small relatively dull pieces that are harmless instead of into larger sharp slivers as is the case with annealed glass.
Opposed blastheads such as of the type disclosed by U.S. Pat. No. 3,936,291 are conventionally utilized to supply quenching air during glass sheet tempering. Such tempering is conventionally performed on flat glass sheets to provide architectural glass, such as in the manner disclosed by U.S. Pat. Nos. 3,806,312, 3,907,132, 3,934,970, 3,947,242, and 3,994,711. In addition, glass sheets are conventionally bent and then tempered between opposed blastheads to provide vehicle glass, such as in the manner disclosed by U.S. Pat. No. 4,282,026.
Relatively thin glass sheets are much more difficult to temper than thicker ones because the surfaces must be cooled very rapidly in order to set up the thermal gradient between the surfaces and the center. Usually the problem becomes significant when the glass has a thickness of 1/8 inch (i.e. about 3 mm) or less.
The rapid cooling necessary to temper thin glass is conventionally provided by supplying the quenching air to the opposed blastheads at a much greater pressure than is utilized with thicker glass sheets. Substantial energy is thus required to pressurize the quenching air in order to temper thin glass, especially at facilities located at high altitudes where the air is much less dense than at lower altitudes. In addition, air quenching of thin glass is much more noisy than for thicker glass due to the high pressure of the quenching air used.